It is known that so-called regenerative braking using a regenerative braking force of the rotating electrical machine is performed on deceleration demand of this type of hybrid vehicle. In the regenerative braking, a rotational speed of the rotating electrical machine decreases with deceleration of the vehicle.
On the other hand, in a configuration in which the stepped transmission is provided between the rotating electrical machine and the axle, it is possible to hold the rotational speed of the rotating electrical machine in a rotational speed region in which the rotating electrical machine has good output characteristics, for a longer time, by sequentially changing a shift position of the stepped transmission to a lower-speed side shift position.
Here, if the shift position is changed in the regenerative braking, the transmission needs to be in a state in which torque does not act. Therefore, during a period in which the shift position is changed, it is necessary to temporarily eliminate the torque of the rotating electrical machine from an input shaft of the transmission, for example, by reducing regenerative torque to a value corresponding to zero-torque or by detaching the rotating electrical machine from the input shaft.
By the way, if the torque of the rotating electrical machine is eliminated from the input shaft of the transmission in this manner, a braking force obtained by the regenerative torque, i.e. the so-called regenerative braking force, is also brought sufficiently close to zero. As a result, although the deceleration is demanded, a reduction in deceleration of the vehicle occurs, which causes a driver to have discomfort and anxiety in some cases.
For such problems, according to a hybrid vehicle disclosed in a patent document 1, if a downshift occurs during the deceleration caused by the regenerative braking force, the regenerative braking force is replaced by a friction braking force caused by a friction brake. Thus, it is considered that it is possible to suppress a variation in the deceleration of the vehicle.
Incidentally, a patent document 2 discloses a technology for a compensating torque reduction in the case where the downshift occurs in the regeneration in a configuration in which a dual clutch transmission (DCT) is provided, by using the friction brake or engine brake. Moreover, it is considered that an engine is started by clutch engagement if the engine brake is used.
Moreover, a patent document 3 discloses a system in which a connection state between a motor output shaft and a transmission input shaft can be changed between IN connection and OUT connection. Moreover, in the configuration, it is also disclosed that the engine is started by inputting motor torque to the engine through a clutch in the IN connection state.